"We had quite a lot of people from the offices who were inconvenienced as, of course, were members of the public."

Officers have advised anyone finding what they believe to be explosive material not to touch it and to leave it where it is, before phoning the police.

Sgt Smith added, "I think he was mighty relieved that it wasn't indeed a high explosive, but he's highly embarrassed about it."

Earlier this year three streets in a town near Newport were evacuated when a pair of unexploded World War II shells were found under the stairs of a terraced house.

Two brothers found the 16in devices as they were renovating the property, which has been empty for years.

All residents living within 50m of the house in Bailey Street, Brynmawr, were evacuated from their homes, and nearby streets were sealed off by police while the army's bomb disposal unit dealt with the anti-aircraft shells.

And only last week a cache of dozens of unexploded shells were found on a building site in Treforest, Pontypridd.

Up to 50 houses in James Place, Windsor Road, Lawn Terrace and Lewis Street were evacuated after a shell was uncovered by contractors working on the site of the Old Foundry at Lawn Terrace.

A subsequent search of the site uncovered more than 30 shells in all.

The military carried out a controlled explosion of one device and removed 33 others.